1. Field of the Invention
The present invention relates to an RF device mainly used in a high frequency radio apparatus, such as a cellular phone.
2. Related Art of the Invention
Recently, as mobile communication users have been increased and a system therefor has become global, an RF device has become a focus of attention that enables the EGSM, DCS and UMTS systems provided for respective frequencies shown in FIG. 18 to be used with one cellular phone. With reference to drawings, a first conventional RF device will be described below.
FIG. 19 is a cross-sectional view of the first conventional RF device. In FIG. 19, reference numeral 1101 denotes a low temperature cofired ceramic body with a low relative dielectric constant. Reference numeral 1102 denotes a multilayered wiring conductor for constituting part of an RF circuit. Reference numeral 1103 denotes an interlayer via hole and reference numeral 1104 denotes a discrete component, such as a discrete resistor, a discrete capacitor, a discrete inductor and a packaged semiconductor.
FIG. 20 is a circuit diagram of the first conventional RF device. The RF device is one provided for triple bands (EGSM, DCS and UMTS described above) comprising a diplexer 1201 that connects a transmitting/receiving switching circuit 1202 and a transmitting/receiving switching circuit 1203 to an antenna (ANT).
An operation of the first conventional RF device arranged as described above will be described.
The multilayered wiring conductor 1102 electrically interconnects a plurality of discrete components 1104 and, in a substrate 1101 made of a low temperature cofired ceramic, forms a capacitor formed in the substrate and an inductor formed in the substrate. Such capacitor and inductor constitute an RF circuit in conjunction with the discrete components 1104, and the RF circuit serves as an RF device such as an RF multilayered switch.
The diplexer 1201 directly connected to the antenna terminal (ANT) branches a signal received through the antenna terminal (ANT) to the transmitting/receiving switching circuits 1202 and 1203. The duplexer 1204 is connected to the transmitting/receiving switching circuit 1203. The transmitting/receiving switching circuit 1202 has a transmitting terminal Tx1 for EGSM transmitting and a receiving terminal Rx1 for EGSM receiving. The transmitting/receiving switching circuit 1203 has a transmitting terminal Tx2 for DCS transmitting and a receiving terminal Rx2 for DCS receiving. The duplexer 1204 has a transmitting terminal Tx3 for UMTS transmitting and a receiving terminal Rx3 for UMTS receiving.
The receiving terminal Rx2 is connected to the antenna via a diode 1205, which is in the off state during transmission using the transmitting terminal Tx2.
Transmission line 1206a and 1206b for electrical length correction, a transmitting filter 1207 and a receiving filter 1208, which are required for duplex transmission, are connected between the transmitting terminal Tx3 and the receiving terminal Rx3.
Now, a second conventional RF device will be described as another example of the send/receive switching circuit directly connected to the antenna.
FIG. 21 is an exploded perspective view of the second conventional RF device. The RF device has six dielectric substrates with high relative dielectric constant 1301a to 1301f. The dielectric substrate 1301b having a shielding electrode 1302a formed on the upper surface thereof, the dielectric substrate 1301c having an inter-stage coupling electrode 1303 formed on the upper surface thereof, the dielectric substrate 1301d having resonator electrodes 1304a and 1304b formed on the upper surface thereof, the dielectric substrate 1301e having input/output coupling capacitor electrodes 1305a and 1305b formed on the upper surface thereof, and the dielectric substrate 1301f having a shielding electrode 1302b formed on the upper surface thereof are stacked.
End face electrodes 1306a and 1306b, which are connected to the shielding electrodes 1302a and 1302 to form ground terminals, are provided at the left and right sides of the stacked dielectric substrates. On the rear of the stacked dielectric substrates, there is provided an end face electrode 1307 which is connected to the ground facing the shielding electrodes 1302a and 1302b and a common open end of the microstrip resonator electrodes 1304a and 1304b. An end face electrode 1308, which is provided on the front of the stacked dielectric substrates, is connected to short-circuit ends of the resonator electrodes 1304a and 1304b and to the shielding electrodes 1302a and 1302b. End face electrodes 1309a and 1309b at the left and right sides of the stacked dielectric substrates are connected to the input/output coupling electrodes 1305a and 1305b to constitute input/output terminals.
FIG. 22 is a circuit diagram of the second conventional RF device. The input/output coupling electrode 1305a and the resonator electrode 1304a constitute an input/output coupling capacitor 1401a, and the input/output coupling electrode 1305b and the resonator electrode 1304b constitute an input/output coupling capacitor 1401b. In addition, the input/output coupling electrode 1305a and the inter-stage coupling electrode 1303 constitute an inter-stage coupling capacitor 1402a, and the input/output coupling electrode 1305b and the inter-stage coupling electrode 1303 constitute an inter-stage coupling capacitor 1402b. These components constitute a two-stage band-pass filter shown in FIG. 22.
FIG. 23 is a block diagram of an antenna duplexer 1503, which is the second conventional RF device, comprising a transmitting filter 1501, a receiving filter 1502, the filters being constituted by the band-pass filter, and a matching circuit provided therebetween.
However, the first conventional RF device configured as described above, the transmitting filter 1206 and the receiving filter 1027 are composed of an inductor or capacitor with a low Quality factor, and therefore, have a high loss as a filter. Furthermore, the microstrip resonator structure for increasing the Quality factor has a problem in that the RF device including the substrate 1101 made of a low temperature cofired ceramic with low relative dielectric constant becomes quite large because the size of the resonator is inversely proportional to the frequency and the square root of the relative dielectric constant.
Even with the microstrip resonator structure, since it is also affected by the substrate 1101 with low relative dielectric constant, the Quality factor cannot be increased sufficiently, and for example, a circuit provided for the CDMA mode still has a problem of the filter loss.
In the second conventional RF device configured as described above, if a line is provided thereon or therein, the impedance of the line is increased because the substrates constituting the RF multilayered device are made of a low temperature cofired ceramic with high relative dielectric constant, and thus, it is quite difficult to form a complicated circuit in each substrate. In addition, it is also quite difficult to implement a discrete component, such as a discrete resistor, a discrete capacitor, a discrete inductor and a packaged semiconductor, on the second conventional RF device, because the line impedance of the discrete component itself is increased.